Munching on Walnuts Is an Easy Way to Boost Gut Health, Study Says

It looks like walnuts are having their moment. While this popular tree nut is already known to improve heart heath and reduce the risk of colon cancer, researchers discovered that these amazing benefits may stem from the effect walnuts have on the trillions of bacteria living inside your gut.

According to a small study published in a May 2018 edition of The Journal of Nutrition, researchers gathered 18 healthy male and female adults and asked them to incorporate either no walnuts or 42 grams (which is equal to about one palm-full) of walnuts into their diet for two three-week-long trials. In order to examine the body’s response to this high-fiber food, the participants’ blood and fecal samples were collected before and after each test period.

The results? Adults in the walnut group showed between a 49-percent and 160-percent increase in three types of gut bacteria: Faecalibacterium (which has been shown in animal studies to reduce inflammation), Roseburia (which may be associated with weight loss, as well as reduced glucose intolerance) and Clostridium (which have the ability to makes butyrate, a short chain fatty acid that encourages the digestive system to function properly).

In addition, the volunteers who were instructed to enjoy walnuts displayed a reduction in secondary bile acids — acids that could damage the gastrointestinal tract that have been found in higher quantities in patients diagnosed with colorectal cancer — as well as LDL-cholesterol (the "bad” cholesterol) levels.

"It is about getting to the 'black box' that is all the microbes in our GI tract to see how they are interfacing with the food we eat and having downstream health effects,” stated lead study author Hannah Holscher, assistant professor of food science and human nutrition at the University of Illinois in a press release. (It was noted in the research that funding for the study was provided by USDA-ARS and California Walnut Commission.)

“I’m not really surprised to hear that walnuts have a positive effect on the microbiome,” says Frances Largeman-Roth, RDN, nutrition expert and author of Eating in Color: Delicious, Healthy Recipes For You and Your Family($13.99, Amazon). “We’ve known for some time that other plant foods, such as pulses, bananas, onions, kiwis and other fruits and vegetables have a beneficial impact on the gut, thanks to their prebiotic attributes. And research continues to shed light on the fascinating topic of gut health.”

As for how many walnuts should be consumed on a daily basis, she refers to the current study, where the researchers used 1.5 ounces of walnuts per day for three weeks. “And that’s about one-third of a cup (18-21 walnut halves), which is also the amount that has been shown to reduce the risk of coronary heart disease,” adds Largeman-Roth. “This amount can definitely fit into a healthy diet, especially since we know that only about 80 percent of the energy in walnuts is absorbed — while the rest goes to those hungry little microbes.”